Method of manufacturing of a monolithic silicon acceleration sensor

ABSTRACT

A method of manufacturing a monolithic silicon acceleration sensor is disclosed. The monolithic silicon acceleration sensor is micromachined from silicon to form one or more sensor cells, each sensor cell having an inertial mass positioned by beam members fixed to a silicon support structure. A sandwiched etch-stop layer is formed between a first silicon wafer section and a second silicon wafer section. A first section of the inertia mass and beam members are formed by etching a U-shaped channel and a bar-shaped channel in the first wafer section of the sandwiched layer to the etch-stop layer. A second section of the inertial mass is formed by aligning a frame-shaped channel in the second wafer section with the U-shaped channel and the bar-shaped channel in the first section, and etching the frame-shaped channel to the etch-stop layer. After stripping exposed etch-stop layer, an inertial mass positioned by beam members fixed to a silicon support structure is formed. A first cover plate structure is bonded to a first surface of the silicon support structure.

FIELD OF THE INVENTION

[0001] This invention relates to acceleration sensors micromachined fromsilicon and, more particularly, sensors having an inertial masspositioned by torsional or cantilever support members.

BACKGROUND OF THE INVENTION

[0002] It is known in the art that small compact acceleration sensorsmay be formed by micromachining silicon wafers into suitableconfigurations that are capable of detecting acceleration along oneaxis. The micromachining process is normally performed on batches ofsilicon wafers. This process consists of masking and forming patterns ofetch stop material on a wafer surface, etching the exposed silicon,removing the etch stop material, metallizing, and bonding. The siliconwafers are diced into individual acceleration sensor devices which arepackaged and connected to suitable electronic circuitry to formaccelerometers. Using these techniques, a two axis or three axisacceleration sensor requires two or three discrete diced devices,respectively, to be precisely mechanically aligned along two or threeorthogonal axes of acceleration. Examples of acceleration sensors formedby a micromachining process are described in the following U.S. Pat.Nos. 4,574,327; 4,930,043; and 5,008,774.

[0003] Prior forms of silicon acceleration sensors employ an inertialmass which moves in response to acceleration, positioned by cantileversupport members that may introduce an asymmetry that can result in anundesirable cross-axis sensitivity. To avoid this undesirable asymmetriceffect, these devices are designed with flexible support members aroundthe periphery of an inertial mass so that the response to accelerationis preferentially along an axis perpendicular to the plane of theinertial mass and the support members. To further limit the accelerationresponse to one axis, the support members are sometimes placed in themid-plane of the inertial mass or symmetrically placed at the top andbottom surfaces of the inertial mass. The devices fabricated in thismanner may exhibit wide parameter variations between devices.Furthermore, for multiple axes applications, multiple discrete devicesmust be precisely aligned mechanically to each axis of acceleration.Difficulties encountered in the fabrication include the accuratelocation of the mid-plane and precise alignment of multiple devices,making the fabrication process complex, slow and expensive.

[0004] For the foregoing reasons, there is a need for a monolithicmultiple axes acceleration sensor micromachined from silicon by arelatively simple fabrication process that results in low mechanicalstress, temperature stable devices with tight parameter tolerancesbetween devices. It is desirable that any required multiple axesalignment be performed as a part of the lithographic process used in thedevice fabrication rather than require precise mechanical alignment ofdiscrete devices after the dicing operation. It is further desirablethat the fabrication process be adjustable on a batch basis, in order toproduce devices with predetermined acceleration sensitivity, withbatches ranging from low sensitivity devices to high sensitivitydevices.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a low mechanical stress,temperature stable, monolithic multiple axes acceleration sensor withtight parameter tolerances between devices that is micromachined fromsilicon by a relatively simple fabrication process. Because the presentmonolithic multiple axes acceleration sensor may be aligned by thelithographic process used in device fabrication, the need for precisemechanical alignment of discrete sensor devices along orthogonal axes ofacceleration is eliminated. The fabrication process of the presentinvention may be adjustable on a batch basis, in order to producedevices with predetermined acceleration sensitivity, with batchesranging from low sensitivity devices to high sensitivity devices.

[0006] Where prior forms of silicon acceleration sensors attempted toavoid asymmetric cross-axis sensitivity, the present invention exploitsthis cross-axis effect to enable fabrication of a monolithic multipleaxes acceleration sensor. The present silicon acceleration sensorinvention comprises one, two, three or four silicon acceleration sensorcells, where each sensor cell comprises a movable silicon inertial massthat moves in response to acceleration and is positioned by beam memberscoplanar with a first surface of the silicon inertial mass and fixed toa silicon support structure. A means is provided for detecting movementof the inertial mass or resulting flexure of the beam members due toacceleration of the inertial mass and the silicon support structure. Therelative position of each inertial mass is at right angles to anadjacent inertial mass when viewing the first surface of each siliconmass, using the position of the beam members as angular reference. Asilicon acceleration sensor device embodying the present inventionhaving a single sensor cell comprising one movable silicon inertial masscan sense acceleration in two orthogonal axes but cannot distinguishbetween acceleration along one axis or the other. A device having twosensor cells, where each sensor cell comprises a movable siliconinertial mass positioned at a 180 degree angle to the inertial mass ofthe other sensor cell when viewing the first surfaces of the inertialmasses using the beam members as an angular reference, can senseacceleration in two orthogonal axes and can distinguish betweenacceleration along both axes. A device having three sensor cells, whereeach sensor cell comprises a movable silicon inertial mass positioned atangles of 0, 90, and 180 degrees relative to each other when viewing thefirst surfaces of the inertial masses using the beam members as anangular reference, can sense acceleration along three orthogonal axesand can distinguish between acceleration along each of the three axes. Adevice having four sensor cells, where each sensor cell comprises amovable silicon inertial mass positioned at angles of 0, 90, 180, and270 degrees relative to each other when viewing the first surfaces ofthe inertial masses using the beam members as an angular reference, cansense acceleration along three orthogonal axes and can distinguishbetween acceleration along each of the three axes. The device comprisingfour sensor cells is of a physically symmetrical geometry when viewingthe first surface of each inertial mass, and provides the capability forcancellation of opposing direction non-linearities. Thus, multiple axesacceleration sensing is achievable with a single monolithic device thatdoes not require precise mechanical alignment of multiple discretesingle axis acceleration sensing devices. One means for detectingmovement of the inertial mass is by measuring the capacitance betweenthe first surface of the movable inertial mass and a first electricallyconductive layer spaced from the first surface and fixed in reference tothe supporting silicon structure; and by measuring the capacitancebetween a second surface of the movable inertial mass opposite the firstsurface and a second electrically conductive layer spaced from thesecond surface and fixed in reference to the supporting siliconstructure. Another means for detecting movement of the inertial massesis by measuring the resistance of piezoresistive elements placed on thepositioning beam members. The beam members may be either in a cantileveror torsion configuration. The shape of the inertial mass is generallydescribed as being a rectangular parallelapiped in the preferredembodiment of the invention.

[0007] A method of manufacture of a silicon acceleration sensor device,having a single silicon acceleration sensor cell with an electricallyconductive silicon movable inertial mass, comprises the forming of alayered sandwich of an etch-stop layer between a first wafer section ofelectrically conductive silicon and a second wafer section ofelectrically conductive silicon, the first wafer section of siliconhaving an exposed first surface and the second wafer section of siliconhaving an exposed second surface. A second section of the siliconinertial mass is formed by etching a rectangular frame-shaped channel inthe second wafer section from the exposed second surface extending tothe etch-stop layer. A first section of the silicon inertial mass isformed by etching a U-shaped channel and a bar-shaped channel in thefirst wafer section from the exposed first surface extending to theetch-stop layer, positioning the bar-shaped channel and the U-shapedchannel in the first wafer section to be in horizontal alignment with,and of equal planar dimensions to the rectangular frame-shaped channelin the second wafer section. Means are provided to electrically connectthe second section of the inertial mass to the first section of theinertial mass through the etch-stop layer or on the etched surface ofthe inertial mass. The silicon dioxide layer that is exposed by theetched frame-shaped channel, the etched U-shaped channel, and the etchedbar-shaped channel is then stripped away, thereby creating a rectangularparallel piped-shaped movable silicon inertial mass positioned by beammembers fixed to a silicon support structure. An alternative means ofelectrically connecting the second section of the inertial mass to thefirst section of the inertial mass is to deposit a layer of conductivepolysilicon over the resulting etched and stripped structure. Thisdeposition process could also be used where it is desired to usenonconductive silicon wafer sections. Means are provided to detectmovement of the silicon inertial mass by fixing a first electricallyconductive layer, spaced from the first surface of the inertial mass,relative to the silicon support structure for a first capacitancemeasurement between the first surface of the inertial mass and the firstelectrically conductive layer; and by fixing a second electricallyconductive layer, spaced from the second surface of the inertial mass,relative to the silicon support structure for a second capacitancemeasurement between the second surface of the inertial mass and thesecond electrically conductive layer. These electrically conductivelayers are preferably metallic in composition. Alternatively, means fordetecting movement of the inertial mass may be provided by placingpiezoresistive elements on the positioning beam members fixed to thesilicon support structure, and measuring the change in resistance whenthe beam members are flexed or twisted.

[0008] In the preferred embodiment of a method of manufacture of asilicon acceleration sensor having at least one silicon accelerationsensor cell, the first section of the movable silicon inertial mass isformed by etching a U-shaped channel and a bar-shaped channel in thefirst layer of silicon from the exposed first surface extending to thesilicon dioxide layer, positioning the bar-shaped channel and theU-shaped channel in the first layer of silicon to be in horizontalalignment with, and of equal planar dimensions to the rectangularframe-shaped channel in the second layer of silicon. The bar-shapedchannel is positioned across the open top of the U-shaped channel,centered within the outside dimensions of the open top of the U-shapedchannel, and extending in length to equal the entire outside width ofthe top of the U-shaped channel. The ends of the bar-shaped channel arespatially separated from the top of the U-shaped channel, so that thespatial separation results in a device with a silicon inertial masspositioned by torsion beam members.

[0009] Although the preferred embodiment of the invention uses a silicondioxide layer as an etch-stop layer, there are alternative embodiments.These alternative embodiments include a layer of silicon nitride, alayer of doped silicon, and the depletion layer associated with thejunction of two differently doped silicon sections.

[0010] In alternative embodiments of a method of manufacture of asilicon acceleration sensor having at least one silicon accelerationsensor cell, the first section of the movable silicon inertial mass isformed by etching a U-shaped channel and a bar-shaped channel in thefirst layer of silicon from the exposed first surface extending to thesilicon dioxide layer, positioning the bar-shaped channel and theU-shaped channel in the first layer of silicon to be in horizontalalignment with, and of equal planar dimensions to the rectangularframe-shaped channel in the second layer of silicon. The bar-shapedchannel is positioned across the open top of the U-shaped channel,centered within the inside dimension of the open top of the U-shapedchannel, and extending in length to be less than the inside width of thetop of the U-shaped channel. The ends of the bar-shaped channel arespatially separated from the inside top of the U-shaped channel, so thatthe spatial separation results in a device with a silicon inertial masspositioned by cantilever beam members.

[0011] A further embodiment of a method of manufacture of a siliconacceleration sensor having at least one silicon acceleration sensor cellis to vary the acceleration sensitivity by adjusting the thickness ofthe beam members by adjusting the thickness of the first silicon wafersection, by adjusting the width of the beam members by adjusting thespatial separation between the U-shaped channel and the bar-shapedchannel, or by adjusting the length of the beam members by adjusting thewidth of the etched channels.

[0012] The method of manufacture of a device having two siliconacceleration sensor cells each comprising a movable silicon inertialmass is identical to the method of manufacture of a device having onesilicon acceleration sensor cell comprising one movable silicon inertialmass, except that a second inertial mass is positioned lithographicallyand then physically at a 90, 180 or 270 (which is functionally the sameas 90) degree angle to the first inertial mass when viewing the exposedfirst surface of the silicon masses, using the positioning beams asangular reference. The method of manufacture of a device having threesilicon acceleration sensor cells, each sensor cell comprising a movablesilicon inertial mass, is identical to the method of manufacture of adevice having two silicon acceleration sensor cells, except that thethird inertial mass is positioned lithographically and then physicallyat a 90 degree angle to the second inertial mass and at a 180 degreeangle to the first inertial mass when viewing the exposed first surfaceof the silicon masses, using the positioning beams as angular reference.The method of manufacture of a device having four silicon accelerationsensor cells, each sensor cell having a movable silicon inertial mass,is identical to the method of manufacture of a device having threeacceleration sensor cells, except that a fourth inertial mass ispositioned lithographically and then physically at a 90 degree angle tothe third inertial mass, at a 180 degree angle to the second inertialmass, and at a 270 degree angle to the first inertial mass when viewingthe exposed first surface of the silicon masses, using the positioningbeams as angular reference. In this manner, a monolithic multiple axesacceleration sensor is formed that does not require precise mechanicalalignment of multiple discrete single-axis acceleration sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1A illustrates a partially broken away perspective viewshowing part of a simplified monolithic silicon acceleration sensorcomprising one silicon acceleration sensor cell without a first and asecond cover plate structure, having an inertial mass positioned bytorsional beams fixed to a silicon support structure.

[0014]FIG. 1B illustrates a partially broken away perspective viewshowing part of a simplified monolithic silicon acceleration sensorcomprising one silicon acceleration sensor cell without a first and asecond cover plate structure, having an inertial mass positioned bycantilever beams fixed to a silicon support structure.

[0015]FIG. 2 illustrates a perspective view showing a simplifiedmonolithic multiple axes acceleration sensor without a first cover platestructure and having four silicon acceleration sensor cells, each havingan inertial mass oriented at a different angle when viewing the beammembers in the plane of the X and Y axes, and each inertial masspositioned by torsional beam members fixed to a silicon supportstructure.

[0016]FIG. 3 shows a chart that indicates the direction of movement ofeach of the movable inertial masses shown in FIG. 2 due to accelerationof the acceleration sensor along the three orthogonal axes ofacceleration.

[0017]FIG. 4A and FIG. 4B illustrate two perspective views showing asimplified monolithic multiple axes acceleration sensor without a firstcover plate structure and having three silicon acceleration sensorcells, each having an inertial mass oriented at a different angle whenviewing the beam members in the plane of the X and Y axes, and eachinertial mass positioned by torsion beam members fixed to a siliconsupport structure.

[0018]FIG. 5 illustrates a perspective view showing a simplifiedmonolithic multiple axes acceleration sensor having two siliconacceleration sensor cells, each having an inertial mass oriented at adifferent angle when viewing the beam members in the plane of the X andY axes, and each inertial mass positioned by torsional beam membersfixed to a silicon support structure.

[0019]FIG. 6 illustrates a partially broken away perspective view of amonolithic silicon acceleration sensor comprising one siliconacceleration sensor cell.

[0020]FIG. 7 illustrates a partially broken away perspective viewshowing a simplified electrically nonconductive single cell monolithicsilicon acceleration sensor with piezoresistive elements on cantileverbeam members.

[0021]FIG. 8 depicts an alternative embodiment of a cover platestructure.

[0022]FIG. 9 illustrates a perspective view of a section of anelectrically conductive silicon wafer.

[0023]FIG. 10A illustrates a perspective view of a second silicon wafersection having silicon nitride dots and a first silicon dioxide layer onone surface.

[0024]FIG. 10B illustrates a sectional view of a second silicon wafersection having silicon nitride dots and a first silicon dioxide layer onone surface.

[0025]FIG. 11A illustrates a perspective view of a second silicon wafersection having a second silicon dioxide layer interspersed with siliconmesas on one surface.

[0026]FIG. 11B illustrates a sectional view of a second silicon wafersection having a second silicon dioxide layer interspersed with siliconmesas on one surface.

[0027]FIG. 12A illustrates a perspective view of the wafer section shownin FIG. 9 with a first silicon wafer section bonded to the silicondioxide layer and ground off.

[0028]FIG. 12B illustrates a sectional view of the wafer section shownin FIG. 9A with a first silicon wafer section bonded to the silicondioxide layer and ground off.

[0029]FIG. 13A illustrates a perspective view of a layer of silicondioxide grown on a silicon wafer section.

[0030]FIG. 13B illustrates a partially broken away perspective view of asandwiched layer of silicon dioxide between silicon wafer sectionsformed by an alternative method of manufacture.

[0031]FIG. 13C illustrates a perspective view of a sandwiched layer ofsilicon dioxide between silicon wafer sections for forming analternative method of manufacture.

[0032]FIG. 14A illustrates a perspective view of the layered sandwichshown in FIG. 12A with depressions formed in the first and secondsurfaces.

[0033]FIG. 14B illustrates a sectional view of the layered sandwichshown in FIG. 12B with depressions formed in the first and secondsurfaces.

[0034]FIG. 15A illustrates a perspective view of the formed secondsection of the inertial mass.

[0035]FIG. 15B illustrates a sectional view of the formed second sectionof the inertial mass.

[0036]FIG. 16A illustrates a perspective view of the formed second andfirst sections of the inertial mass.

[0037]FIG. 16B illustrates a sectional view of the formed second andfirst sections of the inertial mass.

[0038]FIG. 17A illustrates a perspective view of the formed inertialmass positioned by the beam members fixed to the silicon supportstructure.

[0039]FIG. 17B illustrates a sectional view of the formed inertial masspositioned by the beam members fixed to the silicon support structure.

[0040]FIG. 18 illustrates a perspective view of a structure used to showthe formation of cantilever beam members.

[0041]FIG. 19 illustrates the attachment of cover plate structures tothe silicon support structure.

[0042]FIG. 20A illustrates a perspective view partially formed coverplate structure showing a wafer section with a silicon dioxide layer.

[0043]FIG. 20B illustrates a perspective view partially formed coverplate structure showing a trenched wafer section with a silicon mesa.

[0044]FIG. 21 illustrates partially broken away perspective view of thepreferred embodiment of a cover plate structure.

[0045]FIG. 22 illustrates a sectional view of a monolithic siliconacceleration sensor having a single sensor cell connected to capacitancemeasuring circuitry.

DETAILED DESCRIPTION OF THE INVENTION

[0046] Referring now to FIG. 1A, there is shown part of a simplifiedmonolithic silicon acceleration sensor 100 comprising one siliconacceleration sensor cell having an electrically conductive movablesilicon inertial mass 300 positioned by torsion beam members 400 fixedto an electrically conductive silicon support structure 200, an X axis510, a Y axis 520 and a Z axis 530. Similarly, FIG. 1B shows part of asimplified monolithic silicon acceleration sensor 100 comprising onesilicon acceleration sensor cell having the electrically conductivemoveable silicon inertial mass 300 positioned by cantilever beam members410 fixed to the electrically conductive silicon support structure 200,the X axis 510, the Y axis 520 and the Z axis 530. Since the preferredembodiment of the present invention utilizes the torsion beam members400 shown in FIG. 1A to position the movable silicon inertial mass 300,FIG. 1A will be used as a reference for the purposes of describing theoperation of the present invention, but it should be understood that thediscussion applies equally as well to the cantilever beam configurationof FIG. 1B. Considering acceleration relative to the Z axis 530, whenthe silicon acceleration sensor 100 is accelerated in the +Z directionalong the Z axis 530, the inertial mass 300 will move in the −Zdirection along the Z axis 530 relative to the silicon support structure200, rotating about the axis formed by torsion beam members 400.Conversely, when the silicon acceleration sensor 100 is accelerated inthe −Z direction along the Z axis 530, the inertial mass 300 will movein the +Z direction along the Z axis 530 relative to the silicon supportstructure 200, rotating about the axis formed by torsion beam members400. Considering acceleration relative to the X axis 510, when thesilicon acceleration sensor 100 is accelerated in the +X direction alongthe X axis 510, the inertial mass 300 will move in the −X directionalong the X axis 510 relative to the silicon support structure 200,rotating about the axis formed by torsion beam members 400. Conversely,when the silicon acceleration sensor 100 is accelerated in the −Xdirection along the X axis 510, the inertial mass 300 will move in the+X direction along the X axis 510 relative to the silicon supportstructure 200, rotating about the axis formed by torsion beam members400. Considering acceleration relative to the Y axis 520, when theacceleration sensor 100 is accelerated in either the +Y or −Y directionalong the Y axis 520, the inertial mass 300 will be prevented fromrotating about the axis formed by the torsion beam members 400 becausethe force on the inertial mass due to acceleration is not radial butrather in alignment to the axis formed by the torsion beam members 400.Thus, the silicon acceleration sensor configuration of FIG. 1A iscapable of sensing acceleration along two orthogonal axis ofacceleration, along the Z axis 530 and along the X axis 510, but cannotdifferentiate between these two axes of acceleration.

[0047] Turning now to FIG. 2, there is shown part of a simplifiedmonolithic multiple axes silicon acceleration sensor 140 comprising foursilicon acceleration sensor cells, each having a movable siliconinertial mass 310, 320, 330, 340. A first cover plate structure is notshown in FIG. 2 in order to view the relative angular positioning of theinertial masses with reference to the beam members. Each of the fourinertial masses is configured similarly to the movable silicon inertialmass 300 shown in FIG. 1A, being positioned by torsion beam members 400fixed to a silicon support structure 240. However, only inertial mass310 is oriented the same as inertial mass 300 shown in FIG. 1A, withrespect to the orientation of the axis of rotation formed by the torsionbeam members 400 in reference to the X axis 510 and the Y axis 520.Correspondingly, inertial mass 310 only responds to acceleration alongthe X axis 510 and the Z axis 530 by rotating about the axis formed bythe torsion beam members 400, similarly to the inertial mass 300 of FIG.1A. The direction of movement of inertial mass 310 resulting from thedirection of acceleration along the three orthogonal axes ofacceleration is shown in the column under the label _310_ in the chartof FIG. 3. By using a similar analysis to that used to determinemovement of the inertial mass in FIG. 1A in response to acceleration ofthe acceleration sensor, the movement of inertial masses 320, 330, 340may be readily determined. The direction of movement of the fourinertial masses 310, 320, 330, 340 of the acceleration sensor 140 shownin FIG. 2 in response to acceleration along the X axis 510, the Y axis520 and the Z axis 530 is indicated in the chart of FIG. 3.

[0048] Considering the chart of FIG. 3, acceleration of the accelerationsensor 140 of FIG. 2 in the +X direction results in the uniquecombination of movement of inertial mass 310 of FIG. 2 in the −Zdirection, inertial mass 330 of FIG. 2 in the +Z direction, and nomovement of inertial masses 320, 340 of FIG. 2. Conversely, accelerationof the acceleration sensor 140 of FIG. 2 in the −X direction results inthe unique combination of movement of inertial mass 310 of FIG. 2 in the+Z direction, inertial mass 330 of FIG. 2 in the −Z direction, and nomovement of inertial masses 320, 340 of FIG. 2. By similarly consideringinertial mass movement in response to acceleration of the accelerationsensor 140 of FIG. 2 in the +Y, −Y, +Z, and −Z direction, it is seenfrom the results shown in FIG. 3 that there is a unique combination ofmovements of the four inertial masses for any combination ofsimultaneous acceleration magnitude and direction along one, two or allof the three orthogonal axes of acceleration. Thus, the accelerationsensor shown in FIG. 2 is capable of simultaneously sensing accelerationmagnitude and direction along three orthogonal axes of accelerationincluding components resulting from off-axis acceleration. Also notethat while FIG. 2 shows a configuration of four inertial massessymmetrically arranged that respond according to FIG. 3, it can be shownthat only three inertial masses are needed to simultaneously distinguishacceleration direction and magnitude along one, two, or three of theorthogonal axes of acceleration, or any combination of off-axiscomponents of acceleration.

[0049]FIG. 4A shows one possible configuration of a simplifiedmonolithic multiple axes silicon acceleration sensor 120 having threeinertial masses 310, 320, 340 positioned by torsion beam members 400fixed to a silicon support structure 220. FIG. 4B shows another possibleconfiguration of a simplified multiple axes monolithic siliconacceleration sensor 130 having three inertial masses, 310, 320, 340,positioned by torsion beam members 400 fixed to a silicon supportstructure 230. The first cover plate structure is not shown in FIG. 4Aand in FIG. 4B in order to view the relative angular positioning of theinertial masses with reference to the beam members.

[0050] It can also be shown that only two inertial masses are requiredto simultaneously distinguish acceleration direction and magnitude alongone or two orthogonal axes of acceleration, as well as off-axiscomponents. FIG. 5 shows a possible configuration of a simplifiedmonolithic silicon acceleration sensor 110 having two inertial masses320, 340 positioned by torsion beam members 400 fixed to a siliconsupport structure 210. The first cover plate structure is not shown inFIG. 5 in order to view the relative angular positioning of the inertialmasses with reference to the beam members.

[0051] Referring now to FIG. 6, FIG. 6 illustrates a partially brokenaway perspective which represents a view of a complete accelerationsensor that was partly shown in FIG. 1 without the cover structure. FIG.6 illustrates a preferred embodiment of a single sensor cell version ofthe present invention by showing a partially broken away perspectiveview of a monolithic silicon acceleration sensor 150 comprising onesilicon acceleration sensor cell. The sensor cell comprises anelectrically conductive silicon movable silicon inertial mass 300 havinga first surface 302 and an opposing second surface 304. The inertialmass 300 is positioned statically by electrically conductive torsionbeam members, not shown in FIG. 6 but shown as torsion beam members 400in FIG. 1A. The torsion beam members are fixed to the electricallyconductive silicon support structure 200, silicon support structure 200having a first surface 202 and an opposite second surface 204. A firstcover plate structure 600 comprises a first metallic layer 640 spacedfrom the first surface 302 of the inertial mass 300, the first metalliclayer 640 being formed on a first insulator 610, preferably glass, fixedto the first surface 202 of the silicon support structure 200. The firstmetallic layer 640 and the first surface 302 of the inertial mass 300form a first variable capacitor of a value that depends on the positionof the inertial mass 300. A second cover plate structure 700 comprises asecond metallic layer 740 spaced from the second surface 304 of theinertial mass 300, the second metallic layer 740 being formed on asecond insulator 710, preferably glass, fixed to the second surface 204of the silicon support structure 200. The second metallic layer 740 andthe second surface 304 of the inertial mass 300 form a second variablecapacitor of a value that depends on the position of the inertial mass300. The magnitude of the acceleration causing movement in the inertialmass 300 is indicated by measuring the magnitude of the differencebetween the first variable capacitor value and the second variablecapacitor value. The preferred means of electrically connecting theinertial mass 300 to capacitive measuring circuitry is by connecting anelectrical lead wire 880 to an electrical bonding pad 870 formed on anexternal surface of the electrically conductive silicon supportstructure 200 which is electrically connected to the electricallyconductive inertial mass 300 through the electrically conductive beammembers. The preferred means of electrically connecting the firstmetallic layer 640 of the first cover plate structure 600 to capacitivemeasuring circuitry is by a third electrically conductive silicon wafersection 620, having a second surface 624, mounted on the first insulator610 and having a first conductive silicon mesa 630 through the firstinsulator 610 in electrical contact with the first metallic layer 640.The electrical lead wire 880 connected to the capacitive measuringcircuitry is also connected to the electrical bonding pad 870 on thesecond surface 624 of the third silicon wafer section 620 thuscompleting the electrical connection to the first metallic layer 640.Similarly, the preferred means of electrically connecting the secondmetallic layer 740 of the second cover plate structure 700 to capacitivemeasuring circuitry is by a fourth electrically conductive silicon wafersection 720, having a second surface 724, mounted on the secondinsulator 710 and having a second conductive silicon mesa 730 throughthe second insulator 710 in electrical contact with the second metalliclayer 740. The electrical lead wire 880 connected to the capacitivemeasuring circuitry is also connected to the electrical bonding pad 870on the second surface 724 of the fourth silicon wafer section 720 thuscompleting the electrical connection to the second metallic layer 740.In this preferred embodiment of the present invention, the shape of thesilicon inertial mass 300 is a rectangular parallelepiped, the firstsurface 302 of the inertial mass 300 being slightly depressed from thefirst surface 202 of the silicon support structure 200 to providedielectric spacing for the first variable capacitor, and the secondsurface 304 of the inertial mass 300 being slightly depressed from thesecond surface 204 of the silicon support structure 200 to providedielectric spacing for the second variable capacitor.

[0052] Alternative embodiments to the present invention include the useof cantilever beams 410 shown in FIG. 1 B to position the inertial mass300 shown in FIG. 6. Another embodiment of the present invention is theforming of piezoresistive elements on the torsion beam members 400 shownin FIG. 1A or on the cantilever beam members 410 shown in FIG. 1B. FIG.7 depicts a simplified single sensor cell embodiment of a siliconacceleration sensor 100 that illustrates a silicon inertial mass 300 ispositioned by silicon cantilever beam members 410 that are fixed to asilicon support structure 200. Piezoresistive elements 420 are bonded tothe beam members 410 and electrically connected in series to electricalbonding pads 870 via metallized interconnections 890. Bonding wires 880connect these piezoresistive elements to resistance measuring circuitryto determine the level of bending in the beam members 410, giving ameasure of the movement of the inertial mass 300, which is also ameasure of the magnitude of the acceleration experienced by the inertialmass 300.

[0053] An alternative embodiment of electrically connecting the firstmetallic layer 640 of FIG. 6 to capacitive measuring circuitry isillustrated in FIG. 8 which shows an alternative cover plate structure650. The alternative cover plate structure 650 comprises an alternativeinsulator 660 having a first surface 662 and an opposing second surface664. A electrical bonding pad 870 is located on the first surface 662and an alternative metallic layer 668 is located on the second surface664 of the insulator 660. A metallized hole 666 is positioned in theinsulator 660 connecting the metallic layer 668 to the bonding pad 870,and an electrical lead wire 880 bonded to the bonding pad 870 isconnected to capacitive measuring circuitry. Two of these alternativestructures form a first cover plate structure 600 and a second coverplate structure 700 shown in FIG. 6. Although the drawings show theinertial mass in the shape of a cube, it may be shaped as a rectangularparallelepiped in order to increase the sensor sensitivity by increasingthe size of the inertial mass.

[0054] Typical dimensions for some components of FIG. 2 may be asfollows: each cube-shaped inertial mass 310, 320, 330, 340 has sides ofbetween about 300 microns to about 400 microns; the beam members 400have a thickness of between about 5 microns to about 10 microns; thespacing between the inertial masses 310, 320, 330, 340, and the supportstructure 240, known as the channel width, is about 20 microns. Atypical silicon acceleration sensor 110 having four inertial masses 310,320, 330, 340 has sides of about 1200 microns. The typical dimensionsgiven are intended to be illustrative of a typical embodiment only, andshould not be construed as limitations on any physical parameters of thedevices.

[0055] Typical dimensions for some components of FIG. 6 may be asfollows: the thickness of the first insulator 610 is about 75 micronsand the thickness of the second insulator 710 is about 75 microns. Thespacing between the first surface 302 of the inertial mass 300 and thefirst cover plate structure 600 is about 1 micron. The spacing betweenthe second surface 304 of the inertial mass 300 and the second coverplate structure is about 1 micron. The thickness of the first metalliclayer 640 is about several angstroms and the thickness of the secondmetallic layer 740 is about several angstroms.

[0056] As discussed above, the configurations shown in FIG. 2, FIG. 4A,FIG. 4B, and FIG. 5, are monolithic silicon acceleration sensor deviceshaving either four, three, or two acceleration sensor cells with thefirst cover plate structure removed in order to view the relativeangular positioning of the inertial masses with reference to the beammembers. The super-positioning of the structure shown in FIG. 6 onto thestructure of the devices shown in FIG. 2, FIG. 4A, FIG. 4B, and FIG. 5illustrates the complete structure of these monolithic devices.

[0057] Turning now to the method of manufacture of the monolithicsilicon acceleration sensor, silicon micro machining technology is usedin the fabrication of the exemplary sensor device 120 shown in FIG. 6,as well as the multiple sensor cell devices depicted in FIG. 2, FIG. 4A,FIG. 4B, and FIG. 5. A multiplicity of these devices will normally bebatch fabricated using silicon wafers. The method of manufacture of themonolithic silicon acceleration sensor may be subdivided into the stepsof (1) forming a layered sandwich of silicon dioxide between two layersof electrically conductive silicon, (2) fabricating movable siliconinertial masses, beam members, and silicon support structures, (3)fabricating first cover plate structures and second cover platestructures, and bonding the first and second cover plate structures tothe silicon support structures, and (4) dicing the resulting structureinto one, two, three, or four sensor cell devices, bonding electricallead wires, and encapsulating the devices. Since step (4) uses methodsthat are conventional and well known in the art, it will not benecessary to provide a detailed description of these procedures. Whilethe description that follows describes the method of manufacture of amonolithic silicon acceleration sensor device having a single sensorcell, it is understood by those skilled in the art that not only can amultiplicity of single sensor cell devices be batch fabricatedconcurrently, but a multiplicity of multiple sensor cell devices usedfor sensing acceleration along several axes can also be batch fabricatedconcurrently. The main distinguishing difference between the multiplesensor cells within a single device is the angular orientation withrespect to each other sensor cell and the electrical connectionconfiguration. Therefore, the following description is focused on thefabrication of a single sensor cell device, since once this isunderstood, it is more easily understood how a multiplicity of multiplesensor cell devices may be fabricated concurrently. Note that thedimensions in the description are typical for the preferred embodimentof the invention and are for illustrative purposes. The actual devicedimensions will vary depending upon the desired device parameters.

[0058] The preferred embodiment of the present invention begins with thefirst step of forming a layered sandwich of silicon dioxide between afirst layer of electrically conductive silicon having an exposed firstsurface and a second layer of electrically silicon having an exposedsecond surface, the first layer of silicon and the second layer ofsilicon being in electrical contact with each other. Consider FIG. 9depicting a section 250 of an second electrically conductive siliconwafer 292 that is typically 400 microns thick. Note that there are alsosimilar first wafer section, third wafer section, and fourth wafersection that are considered in subsequent fabrication steps. The secondwafer section 250, depicted in FIG. 10A, is also typically 400 micronsthick and has a first surface 256 and second surface 258 that aretypically 600 microns square. The preferred method of fabricating thelayered sandwich is by growing dots of silicon nitride 252 on the firstsurface 256 of the second wafer section 250 at locations that will notinterfere with etching operations that will be performed in laterfabrication steps. The first surface 256 of the second wafer section 250is then thermally oxidized which causes a selective first silicondioxide layer 254 to be grown at locations not covered by dots ofsilicon nitride 252. FIG. 10B shows a cross section of the second wafersection 250 of FIG. 10A having silicon mesas 262 resulting from theoxidation process. The first silicon dioxide layer 254 is then strippedoff of the first surface 256 of the second wafer section 250, leaving aformed depression in the first surface 256 of the second wafer section250, relative to the interface between the silicon and the siliconnitride dots 252. FIG. 11A and FIG. 11B depict the second wafer section250 after a second silicon dioxide layer 260 is thermally grown in theformed depression in the second wafer section 250, extending to a levelcorresponding to the interface between the silicon and the siliconnitride dots 252 of FIG. 10B, and the silicon nitride dots 252 arestripped off. Thus, a planar surface is formed near the first surface256 of the second wafer section 250 comprising the second silicondioxide layer 260 interspersed with silicon mesas 262 as shown in FIG.11A and FIG. 11B. FIG. 12A and FIG. 12B illustrate a first wafer section270 of a second electrically conductive silicon wafer having firstsurface 276 and second surface 278 that are typically 600 micronssquare. This first wafer section 270 is bonded to the formed planarsurface of the second wafer section 250, such that the second surface278 of the first wafer section 270 is in contact with the formed planarsurface. The first wafer section 270 is then ground off to a value thatis typically between 5 and 10 microns. The value will determine thethickness of the beam members formed in a later step of fabrication.This results in a layered sandwich of silicon dioxide between a firstsilicon wafer section 270 and a second silicon wafer section 250 havinga typical thickness of about 400 microns, whereby the first wafersection 270 and the second wafer section 250 are electricallyinterconnected through the silicon dioxide layer via the silicon mesas262.

[0059] In addition to the preferred embodiment described above, thereare several alternative embodiments of forming a sandwiched layer ofsilicon dioxide between two layers of silicon. One alternativeembodiment to produce a structure similar to that shown in FIG. 12A andFIG. 12B is by implanting ions to a depth typically of 5 to 10 micronsbelow the surface of the second silicon dioxide layer 260 of the secondwafer section 250 shown in FIG. 11A and FIG. 11B, prior to bonding thefirst silicon wafer section 270 of FIG. 12A and FIG. 12B to the planarsurface of the second silicon dioxide layer 260 on the second wafersection 250, as shown in FIG. 12A and 12B. The first wafer section isnot ground off, as above, but the resulting structure is thermallyshocked. The thermal shock is such that the second wafer section 250 iscaused to be cleaved along the junction of the ion implantation and theremaining silicon of the second wafer section 250, resulting in astructure that is inverted from the structure shown in FIG. 12A and FIG.12B, in that the second wafer section 250 is typically between 5 and 10microns thick and the first wafer section 270 is typically 400 micronsthick. Another alternative embodiment to produce a structure similar tothat in FIG. 12A and FIG. 12B is by growing a first layer of silicondioxide 254 on the second wafer section 250, as shown in FIG. 13A, andthen exposing several small areas 255 of the second wafer section 250through the silicon dioxide layer 254, creating a puddle of moltensilicon, and drawing the puddle of molten silicon onto the exposedsurface of the first silicon dioxide layer 254, as shown in FIG. 13B. Afirst layer of silicon 270 is formed on top of the silicon dioxide layer254 when the molten silicon cools as shown in FIG. 13B, resulting in astructure that is similar to FIG. 12A and FIG. 12B. Another alternativeembodiment to produce a structure similar to FIG. 12A and FIG. 12B isforming a first layer of silicon dioxide 254 on the first surface 256 ofthe second wafer section 250 as shown in FIG. 13A. The second surface278 of the first wafer section 270 is bonded to the first silicondioxide layer 254 as shown in FIG. 13C. A multiplicity of small holes isexposed in either the first wafer section 270 or the second wafersection 250 extending to the silicon dioxide layer 254, stripping theexposed silicon dioxide layer 254, and depositing conductive polysiliconor other conductive material in the small holes. This results in formingelectrical connections between the first wafer section 270 and thesecond wafer section 250.

[0060] The second step of the preferred embodiment is fabricating amovable silicon inertial mass 300, beam members 400, and a siliconsupport structure 200 as depicted in FIG. 1A. The layered sandwich ofthe silicon dioxide layer 260 between the first wafer section 270 andthe second wafer section 250 shown in FIG. 12A and FIG. 12B forms thestarting point for this second step. To provide space for movement of aninertial mass to be formed in subsequent steps, a first one microndepression 284 is formed on the first surface 276 of the first wafersection 270 and a second one micron depression 264 is formed on thesecond surface 258 of the second wafer section 250, as shown in FIG. 14Aand FIG. 14B. Note that the second silicon dioxide layer 260 and thesilicon mesas 262 shown in FIG. 14B were formed in a previous step ofthe fabrication process. The first depression 284 and the seconddepression 264 are formed by growing a first layer of silicon nitride onthe exposed first surface 276 of the first wafer section 270 and asecond layer of silicon nitride on the exposed second surface 258 of thesecond wafer section 250 shown in FIG. 14B. The first layer of siliconnitride and the second layer of silicon nitride are masked to provide afirst and a second exposed rectangular area, for the first rectangulardepression 284 and the second rectangular depression 264. The first andthe second exposed rectangular areas are positioned to be in horizontalalignment with each other. The exposed first and second rectangularareas are then stripped of the silicon dioxide layer so that first andsecond rectangular areas of silicon are exposed on the first wafersection 270 and the second wafer section 250. Layers of silicon dioxideare grown on the exposed silicon in the first and second rectangularareas. The masking on the silicon nitride layers are removed and thesilicon nitride and the silicon dioxide are stripped off, leaving a 1micron depression on the first surface 276 of the first wafer section270 and a 1 micron depression on the second surface 258 of the secondwafer section 250 where the layers of silicon dioxide had been grown, asdepicted in FIG. 14A and FIG. 14B.

[0061] A second section 308 of a movable silicon inertial mass is formedin the second wafer section 250 of FIG. 14B as shown in FIG. 15A andFIG. 15B by masking a rectangular frame-shaped area with a width oftypically 20 microns within the periphery of the second depression 264of FIG. 14B, the rectangular frame shaped area having a major and aminor dimension. A silicon dioxide layer is grown over the remainingexposed area of the second surface 258 of the second wafer section 250shown in FIG. 14B and the frame-shaped masking is removed, exposing aframe-shaped area of silicon within the second depression 264 on thesecond surface 258 of the second wafer section 250 of FIG. 14B. Theexposed silicon is etched, preferably resistive ion etched (RIE), fromthe exposed second surface 258 of FIG. 14B extending to the silicondioxide layer 260 that forms an etch stop, creating a frame-shapedchannel 266 in the second wafer section 250 of FIG. 14B, resulting inthe silicon support structure 200 and the second section 308 of theinertial mass as shown in FIG. 15A and FIG. 15B. Within the channel 266is the second section 308 of the inertial mass having a second surface304 that was previously part of the second surface 258 of the secondwafer section 250 of FIG. 14B. Outside the channel 266 is the siliconsupport structure 200 having a second surface 204 that was previouslypart of the second surface 258 of the second wafer section 250 of FIG.14B. A first section 306 of the movable silicon inertial mass 300 ofFIG. 1A and torsion beam members 400 are formed in the first wafersection 270 of FIG. 14B as shown in FIG. 16A and FIG. 16B by masking aU-shaped area and a bar-shaped area, each with a width of typically 20microns, within the periphery of the first depression 284. Thebar-shaped area has a long dimension that is aligned with the majordimension of the rectangular frame-shaped channel 266 shown in FIG. 15Aand FIG. 15B. The U-shaped area and the bar-shaped area are positionedto be in horizontal alignment with, and of equal planar dimensions tothe rectangular frame-shaped channel 266 of FIG. 15A previously formedin the second wafer section 250 of FIG. 14A. This alignment enables arectangular parallel piped inertial mass to be formed after thesubsequent etching process of the first section 306 inertial mass. Asilicon dioxide layer is grown over the remaining exposed area of thefirst surface 276 of the first wafer section 270 of FIG. 14A, and theU-shaped and bar-shaped masking is removed, exposing a U-shaped and abar-shaped area of silicon within the first depression 284 on the firstsurface 276 of the first wafer section 270 of FIG. 14A. The exposedsilicon is etched, preferably RIE, from the exposed first surface 276extending to the silicon dioxide layer 260 of FIG. 14A that forms anetch stop, creating a U-shaped channel 286 and a bar-shaped channel 288in the first wafer section 250 of FIG. 14A, as shown in FIG. 16A andFIG. 16B. The interstitial silicon between the U-shaped channel 286 andthe bar-shaped channel 288 form the torsion beam members 400. Within theU-shaped channel 286 and the bar-shaped channel 288 is the first section306 of the inertial mass having a first surface 302 that was previouslypart of the first surface 276 of the first wafer section 270 shown inFIG. 14A. Outside the channels 286, 288 is the silicon support structure200 having a first surface 202 that was previously part of the firstsurface 276 of the first wafer section 270 shown in FIG. 14A. Theresulting structure shown on FIG. 16A and FIG. 16B is the inertial mass300 of FIG. 1A held in place by a web of silicon dioxide and the torsionbeam members 400 that are fixed to the silicon support structure 200.The inertial mass shown in FIG. 16B comprises the first section 306 thatwas part of the first wafer section 270 and the silicon dioxide layer260 of FIG. 14B, and the second section 308 that was part of the secondwafer section 250 of FIG. 14B. The silicon support structure 200 shownin FIG. 16B comprises part of the first wafer section 270, the silicondioxide layer 260, and the second wafer section 250 of FIG. 14B.

[0062] The entire structure is stripped of exposed silicon dioxide inthe etched frame-shaped channel 266, in the etched U-shaped channel 286,and in the etched bar-shaped channel 288 as shown in FIG. 16A and FIG.16B, thereby creating a rectangular parallel piped-shaped inertial mass300 having a first surface 302 and a second surface 304, positioned bytorsion beam members 400 fixed to a silicon support structure 200 havinga first surface 202 and a second surface 204, as shown in FIG. 17A andFIG. 17B. The stripping agent used is typically hydrogen fluoride.

[0063] There are several alternatives to the preferred embodiment forfabricating the movable silicon inertial masses 300, beam members 400,and silicon support structures 200 shown in FIG. 17A and FIG. 17B. Oneof these alternatives include adjusting the thickness of the beammembers 400 by adjusting the thickness of the first wafer section 270 ofFIG. 14A, which may be accomplished by either epitaxially growingsilicon onto the exposed first surface 276 of the first wafer section270 or by ion milling or grinding the exposed first surface 276 of thefirst wafer section 270 of FIG. 14A. Other alternative embodiments areto adjust the width of the beam members by adjusting the spatialseparation between the U-shaped channel 286 and the bar-shaped channel288 of FIG. 16A, or to adjust the length of the beam members byadjusting the width of the etched channels 266, 286, 288 as shown inFIG. 16A and FIG. 16B. The preferred embodiment for forming torsion beammembers 400, as shown in FIG. 1A, is, with reference to FIG. 16A andFIG. 16B, by positioning the bar-shaped channel 288 across the open topof the U-shaped channel 286, centering the bar-shaped channel 288 withinthe outside dimension of the U-shaped channel 286, extending the lengthof the bar-shaped channel 288 to equal the entire outside width of thetop of the U-shaped channel 286, and spatially separating the ends ofthe bar-shaped channel 288 from the top of the U-shaped channel 286. Analternative embodiment is the forming of cantilever beam members 410, asshown in FIG. 1B. With reference to FIG. 18, the cantilever beam membersare formed by positioning the bar-shaped channel 288 across the open topof the U-shaped channel 286, centering the bar-shaped channel 288 withinthe inside dimension of the U-shaped channel 286, extending the lengthof the bar-shaped channel 288 to be less than the inside width of thetop of the U-shaped channel 286, and spatially separating the ends ofthe bar-shaped channel 288 from the inside top of the U-shaped channel286.

[0064] An alternative embodiment for electrically connecting first wafersection 270 and second wafer section 250, is to deposit a conductivematerial, preferably polysilicon, on the side walls of the U-shapedchannel 286, the frame shaped channel 266 and the bar shaped channel 288after stripping the exposed silicon dioxide layer 260 within theseareas.

[0065] An alternative embodiment for detecting movement of the inertialmass 300 by fixing piezoresistive elements 420 to the beam members 410requires a simple first cover plate structure 600 and a simple secondcover plate structure 700, both of an insulating material such as glassto be bonded to the silicon support structure 200, as shown in FIG. 7.The piezoresistive elements are then electrically connected to suitableresistance measuring circuitry to determine the amount of twisting orbending of the beam members due to movement of the inertial mass inresponse to acceleration.

[0066] The third step of the preferred embodiment of the invention isfabrication the first cover plate structure 600 and second cover platestructure 700 and bonding the cover plate structures to the siliconsupport structure 200 as shown in FIG. 19. The preferred embodiment fordetecting movement of the inertial mass is by measuring two variablecapacitances. The first variable capacitance is between the firstsurface 302 of the inertial mass 300 and a first metallic layer 640fixed to a first cover plate structure 600 that is insulated from andfixed to the silicon support structure 200. The second variablecapacitance is between the second surface 304 of the inertial mass 300and a second metallic layer 740 fixed to a second cover plate structure700 that is insulated from and fixed to the silicon support structure200. The first cover plate structure 600 is a mirror image of the secondcover plate structure 700 as shown in FIG. 19, so for brevity, only thefabrication of the first cover plate structure will be described.

[0067] Referring to FIG. 20A, the preferred embodiment for fabricatingthe first cover plate structure 600 is by growing a first layer ofsilicon dioxide 626 on an exposed first surface 622 of an electricallyconductive third wafer section 620, the third wafer section 620 having asecond surface 624 opposite the first surface 622. The first layer ofsilicon dioxide 626 on the third wafer section 620 is masked so that thesilicon dioxide surface is exposed except for a small shaped patternthat is positioned to coincide with the location of the inertial mass,as shown in FIG. 20A. The exposed silicon dioxide layer 626 is strippedto expose the silicon of the first surface 622 of the third wafersection 620 except for the masked shaped pattern. The exposed siliconsurface is etched to a depth of typically 75 microns so that a smallsilicon mesa 630 is formed on the first surface 622 of the third wafersection 620 as shown in FIG. 20B. Trenches are then formed in arectangular crosshatched pattern on the first surface 622 of the thirdwafer section 620 to a depth of typically half the thickness of thethird wafer section 620, or about 200 microns. The rectangularcrosshatch contains a silicon mesa and is positioned to coincide withthe position of the inertial mass, as shown in FIG. 20B.

[0068]FIG. 21 is a partially broken away perspective of the first coverplate structure 600 that depicts the trenched third wafer section 620after a layer of glass is melted over the first surface 622 of the thirdwafer section 620 such that the trenches are filled with glass and thesilicon mesa 630 is covered with glass. The glass surface is ground flatforming a planar glass surface 612 having the top of the mesa 630exposed, and the second surface 624 of the third wafer section is background so that the glass-filled trenches are exposed, as shown in FIG.21. A first metallic rectangular pattern layer 640 is formed on theplanar glass surface 612 of the first cover plate structure so that themetallic layer 640 is electrically connected to the oppositeelectrically conductive third wafer section 620 and its second surface624 by the electrically conductive silicon mesa 630. The first metalliclayer 640 is positioned and sized to coincide with the first surface 302of the inertial mass 300 shown in FIG. 19.

[0069] The glass surface 612 of the first cover plate structure 600shown in FIG. 21 is bonded to the first surface 202 of the siliconsupport structure 200 shown in FIG. 19, so that the first metallic layer640 is coincident with and spaced from the first surface 302 of theinertial mass 300, such that a first variable capacitor is formedbetween the first surface 302 and the first metallic layer 640 as shownin FIG. 19. Similarly, the second cover plate structure 700 is bonded tothe second surface 204 of the silicon support structure 200 such that asecond variable capacitor is formed between the second surface 304 ofthe inertial mass and the second metallic layer 740 as shown in FIG. 19.Electrical bonding pads 870 are formed on the second surface 624 of thethird wafer section 620, on the surface of the silicon support structure200, and on the second surface 724 of the fourth wafer section 720, asshown in FIG. 6. Electrical lead wires 880 connect the first cover platestructure 600, silicon support structure 200, and second cover platestructure 700 shown in FIG. 6 to electronic circuitry for measuring thevalue of the first variable capacitor and the value of the secondvariable capacitor, providing a measurement of the movement of theinertial mass 300 which is an indication of the acceleration magnitudeand direction experienced by the sensor. FIG. 22 shows a cross sectionof a monolithic silicon acceleration sensor having a single sensor cellconnected to capacitance measuring electronic circuitry.

[0070] Another embodiment in fabricating an alternate first cover platestructure 650 is forming a small hole 666 in a section of electricallyinsulating material 660 having a first surface 662 and a second surface664 as shown in FIG. 8, such that the hole coincides with the locationof an inertial mass. The surface of the hole 666 is metallized as wellas a first rectangular metallic layer 668 on the second surface 664 ofthe insulating material 660, such that the rectangular metallic layer668 on the second surface 664 is electrically connected to the firstsurface by the metallized hole and is positioned and sized to coincidewith a first surface of an inertial mass. A electrical bonding pad 870is formed on the first surface 662 of the insulating material 660 inelectrical contact with the metallized hole 666. The second surface 664of the insulating material 660 is bonded to the first surface of thesilicon support structure such that the metallized layer 668 iscoincident with and spaced from the first surface of the inertial masswhereby a first variable capacitor is formed. A second cover platestructure is similarly formed and bonded to the second surface of thesilicon support structure.

[0071] It was noted above that monolithic acceleration sensors havingmore than one sensor may be manufactured by the method described aboveby merely changing the angular orientation of the beam members withrespect to each other. FIG. 5 shows a monolithic acceleration sensor 110having a first and a second acceleration sensor cell, whereby the secondsensor cell having an inertial mass 320 is oriented at a 180 degreeangle from the first sensor cell having an inertial mass 340, whenviewing the first surface of the inertial masses, using the beam membersas an angular reference. FIG. 4A and FIG. 4B show alternatives of amonolithic acceleration sensor 120, 130 having a first, a second, and athird acceleration sensor cell, whereby the second sensor cell having aninertial mass 310 is oriented at a 90 degree angle from the first sensorcell having an inertial mass 340 and the third sensor cell having aninertial mass 320 is oriented at a 180 degree angle from the firstsensor cell having an inertial mass 340, when viewing the first surfaceof the inertial masses, using the beam members as an angular reference.FIG. 2 shows a monolithic acceleration sensor 140 having a first, asecond, a third, and a fourth acceleration sensor cell, whereby thesecond sensor cell having an inertial mass 310 is oriented at a 90degree angle from the first sensor cell having an inertial mass 340, thethird sensor cell having an inertial mass 320 is oriented at a 180degree angle from the first sensor cell having an inertial mass 340, andthe fourth sensor cell having an inertial mass 330 is oriented at a 270degree angle from the first sensor cell having an inertial mass 340,when viewing the first surface of the inertial masses, using the beammembers as an angular reference.

[0072] Although the present invention has been described in considerabledetail with reference to certain preferred versions thereof, otherversions are possible. It should be understood that the embodimentsdescribed herein are merely exemplary and that many alternateembodiments and additional embodiments will become apparent to thoseskilled in the art. Accordingly such alternative embodiments are to beconstrued as being within the spirit of the present invention eventhough not explicitly set forth herein, the present invention beinglimited only by the content and scope of claims appended hereto.

I claim:
 1. A method of manufacturing a monolithic silicon accelerationsensor comprising the step of forming at least one silicon accelerationsensor cell, the step of forming a sensor cell comprising the steps of:(a) forming a layered sandwich of an etch-stop layer between a firstwafer section of electrically conductive silicon having an exposed firstsurface and a second wafer section of electrically conductive siliconhaving an exposed second surface; (b) forming a second section of amovable silicon inertial mass by etching a rectangular frame-shapedchannel in the second wafer section of silicon from the exposed secondsurface extending to the etch-stop layer; (c) forming a first section ofthe inertial mass by etching a U-shaped channel and a bar-shaped channelin the first wafer section of silicon from the exposed first surfaceextending to the etch-stop layer, positioning the bar-shaped channel andthe U-shaped channel in the first wafer section of silicon to be inhorizontal alignment with, and of equal planar dimensions to therectangular frame-shaped channel in the second wafer section of silicon;(d) stripping the etch-stop layer that is exposed by the etchedframe-shaped channel, the etched U-shaped channel, and the etchedbar-shaped channel, thereby creating a rectangular parallel piped-shapedinertial mass having a first and a second exposed surface, the inertialmass positioned by beam members fixed to a silicon support structurehaving a first and a second exposed surface; and (e) providing a meansfor detecting movement of the inertial mass.
 2. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of forming the layered sandwich comprises thesteps of: (a) growing a layer of silicon dioxide on one surface of afirst silicon wafer section; and (b) bonding a second silicon wafersection to the surface of the silicon dioxide layer.
 3. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of forming the layered sandwich comprises thesteps of: (a) growing a layer of silicon dioxide on one surface of afirst silicon wafer section; (b) implanting ions below the surface ofthe silicon dioxide layer into the first silicon wafer section; (c)bonding a second silicon wafer section to the surface of the silicondioxide layer; and (d) thermally shocking the resulting structure suchthat the first silicon wafer section is caused to be cleaved along thejunction of the ion implantation and the remaining silicon of the firstwafer section.
 4. The method of manufacturing a monolithic siliconacceleration sensor according to claim 1, wherein the step of formingthe layered sandwich comprises the steps of: (a) growing a layer ofsilicon dioxide on one surface of a silicon wafer section; (b) exposinga small area of the silicon wafer section through the silicon dioxidelayer; (c) thermally creating a puddle of molten silicon; and (d)drawing the puddle of molten silicon onto the exposed surface of thesilicon dioxide layer from the exposed area of the silicon wafer sectionsuch that a second wafer section of silicon is formed on the silicondioxide layer when the molten silicon cools.
 5. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of forming a layered sandwich furthercomprises: (a) creating a multiplicity of electrical interconnectionpaths between the first wafer section of silicon and the second wafersection of silicon through the etch-stop layer; and (b) patterning theinterconnection paths in such a way that they will not interfere withany subsequent etching steps.
 6. The method of manufacturing amonolithic silicon acceleration sensor according to claim 5, wherein thestep of creating a multiplicity of electrical interconnection pathscomprises the steps of: (a) forming a multiplicity of small holes in theeither the first wafer section of silicon or the second wafer section ofsilicon, extending to the etch-stop layer; (b) removing the etch-stoplayer exposed at the bottom of the small holes; (c) depositing aconductive material such as polysilicon in the small holes, so thatelectrical connection is made between the first silicon wafer sectionand the second silicon wafer section through the small holes.
 7. Themethod of manufacturing a monolithic silicon acceleration sensoraccording to claim 5, wherein the step of creating a multiplicity ofelectrical interconnection paths comprises the steps of: (a) forming amultiplicity of small holes in either the first wafer section of siliconor in the second wafer section of silicon, extending to the etch-stoplayer; (b) removing the etch-stop layer exposed at the bottom of thesmall holes; and (c) metallizing the surfaces of the holes, so thatelectrical connection is made between the first and second silicon wafersections through the small metallized holes.
 8. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of forming a layered sandwich comprises thesteps of: (a) growing dots of silicon nitride on a first surface of asecond silicon wafer section in a pattern such that the dots will notinterfere with any subsequent etching operations; (b) growing a firstlayer of silicon dioxide on the remaining exposed silicon on the firstsurface of the second silicon wafer section; (c) stripping off the firstsilicon dioxide layer so that a depression is formed in the firstsurface of the second silicon wafer section relative to the interfacebetween the silicon and the dots of silicon nitride, thereby formingsilicon mesas beneath the silicon nitride dots; (d) growing a secondsilicon dioxide layer in the formed depression extending to a levelcorresponding to the interface between the silicon mesas and the siliconnitride dots; (e) stripping off the silicon nitride dots such that aplanar surface is formed by the second silicon dioxide layerinterspersed with silicon mesa tops; (f) bonding a first silicon wafersection to the formed planar surface of silicon dioxide interspersedwith silicon mesa tops, such that the layered sandwich is formed wherethe first and second silicon wafer sections are electricallyinterconnected through the silicon dioxide layer via the mesas ofsilicon; and (g) adjusting the thickness of the of the first siliconwafer to correspond to the desired thickness of the beam members bygrinding off the first silicon wafer section.
 9. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein: (a) the step of forming the second section of aninertial mass is by dry etching a rectangular frame-shaped channel inthe second wafer section of silicon extending from the exposed secondsurface to the etch-stop layer, the rectangular frame-shaped channelhaving a major and a minor dimension; and (b) the step of forming thefirst section of an inertial mass comprises the steps of dry etching aU-shaped channel and a bar-shaped channel having a long dimension in thefirst wafer section of silicon, the channel extending from the exposedfirst surface to the etch-stop layer, the long dimension of thebar-shaped channel is aligned with the major dimension of therectangular frame-shaped channel in the second wafer section of silicon,thus providing the capability of increasing the inertial mass by anincrease in the major dimension without changing the inertial balance inthe plane normal to the major dimension.
 10. The method of manufacturinga monolithic silicon acceleration sensor according to claim 1, whereinthe step of forming a second section of the inertial mass is by dryetching a square frame-shaped channel extending from the exposed secondsurface to the etch-stop layer, the square frame-shape channel havinginside dimensions substantially equal to the thickness of the layeredsandwich, so that a cube shaped inertial mass positioned by beam membersis formed.
 11. The method of manufacturing a monolithic siliconacceleration sensor according to claim 1, further comprising the stepsof: adjusting the thickness of the beam members by adjusting thethickness the first wafer section of silicon; (g) adjusting the width ofthe beam members by adjusting the spatial separation between theU-shaped channel and the bar-shaped channel; and (h) adjusting thelength of the beam members by adjusting the width of the etchedchannels.
 12. The method of manufacturing a monolithic siliconacceleration sensor according to claim 11, wherein the step of adjustingthe thickness of the beam members is performed by epitaxially growingsilicon onto the exposed first surface of the first wafer section ofsilicon to a desired thickness.
 13. The method of manufacturing amonolithic silicon acceleration sensor according to claim 11, whereinthe step of adjusting the thickness of the beam members is performed bymilling the exposed first surface of the first wafer section of siliconto a desired thickness.
 14. The method of manufacturing a monolithicsilicon acceleration sensor according to claim 1, wherein: (a) the stepof forming the second section of the inertial mass further comprisescreating a rectangular second depression on the exposed second surfaceof the second wafer section of silicon prior to etching the rectangularframe-shaped channel within the second depression; and (b) the step offorming the first section of the inertial mass further comprisescreating a rectangular first depression on the exposed first surface ofthe first wafer section of silicon prior to etching the U-shaped channeland the bar-shaped channel within the first depression.
 15. The methodof manufacturing of a monolithic silicon acceleration sensor accordingto claim 14, wherein the step of creating the rectangular firstdepression and the rectangular second depression comprises the steps of:(a) rowing a first layer of silicon nitride on the exposed first surfaceof the first wafer section of silicon and a second layer of siliconnitride on the exposed second surface of the second wafer section ofsilicon; (b) asking the first silicon nitride layer and the secondsilicon nitride layer thereby providing a first and a second exposedrectangular area for the rectangular depressions on the first layer ofsilicon nitride and on the second layer of silicon nitride; (c)positioning the first and the second rectangular areas to be inhorizontal alignment with each other; (d) tripping the exposed first andsecond rectangular areas of silicon nitride so that first and secondrectangular areas are exposed on the first wafer section of silicon andon the second wafer section of silicon, respectively; (e) growing alayer of silicon dioxide on the exposed first and second rectangularareas of silicon; (f) removing the masking on the silicon nitridelayers; and (g) stripping the exposed silicon nitride and the exposedsilicon dioxide from the first and the second silicon wafer sections.16. The method of manufacturing of a monolithic silicon accelerationsensor according to claim 1, wherein the step of forming the firstsection of the inertial mass comprises the steps of: (a) positioning thebar-shaped channel across the open top of the U-shaped channel andcentering the bar-shaped channel within the outside dimension of theopen top of the U-shaped channel; (b) extending the length of thebar-shaped channel to equal the entire outside width of the open top ofthe U-shaped channel; and (c) spatially separating the ends of thebar-shaped channel from the open top of the U-shaped channel, so thatthe inertial mass is positioned by torsion beam members.
 17. The methodof manufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of forming the first section of the inertialmass comprises the steps of: (a) positioning the bar-shaped channelacross the open top of the U-shaped channel and centering the bar-shapedchannel within the inside dimension of the open top of the U-shapedchannel; (b) extending the length of the bar-shaped channel to be lessthan the inside width of the open top of the U-shaped channel; and (c)spatially separating the ends of the bar-shaped channel from the insideopen top of the U-shaped channel, so that the silicon inertial mass ispositioned by cantilever beam members.
 18. The method of manufacturing amonolithic silicon acceleration sensor according to claim 1, wherein thestep of providing a means to detect movement of the inertial masscomprises the steps of: (a) measuring the capacitance between the firstsurface of the inertial mass and a first electrically conductive layerspaced from the first surface of the inertial mass, insulated from andfixed to the silicon support structure; and (b) measuring thecapacitance between the second surface of the inertial mass and a secondelectrically conductive layer spaced from the second surface of theinertial mass, insulated from and fixed to the silicon supportstructure.
 19. The method of manufacturing a monolithic siliconacceleration sensor according to claim 1, wherein the step of providinga means to detect movement of the inertial mass comprises the steps of:(a) growing a first layer of silicon dioxide on an exposed first surfaceof an electrically conductive third wafer section of silicon and a firstlayer of silicon dioxide on an exposed first surface of an electricallyconductive fourth wafer section of silicon, the third and fourth siliconwafer sections having a second surface opposite the exposed firstsurface; (b) masking the first silicon dioxide layer on the third and onthe fourth silicon wafer sections so that the silicon dioxide surface isexposed except for a small post-shaped pattern such that the post-shapedpattern is positioned to coincide with the location of the inertialmass; (c) stripping the silicon dioxide layer from the exposed silicondioxide surfaces so that exposed areas of silicon are formed except fora post-shaped pattern of masked silicon dioxide on the first surface ofthe third and the fourth silicon wafer sections; (d) etching the exposedareas of the third and the fourth silicon wafer sections to a depth ofabout 75 microns so that a small mesa of silicon is formed on the firstsurface of the third silicon wafer section beneath the post-shapedpattern and on the first surface of the fourth silicon wafer sectionbeneath the post-shaped pattern; (e) forming trenches on the firstsurface of the third and the fourth silicon wafer sections in arectangular crosshatched pattern to a depth of about half of the thirdand fourth silicon wafer section thickness such that an enclosedrectangle contains at lease one silicon mesa and is positioned tocoincide with the position of the inertial mass; (f) melting a layer ofglass over the first surface of the third and the fourth silicon wafersections having the silicon mesa and trenches, such that the trenchesare filled with glass and the silicon mesas are covered with glass; (g)grinding the glass flat so that a planar glass surface is formed on thefirst surface of the third and the fourth silicon wafer sections, theplanar glass surface having a silicon mesa pattern exposed; (h) backgrinding the second surface of the third and the fourth silicon wafersections so that the glass-filled trenches are exposed and electricalisolation is formed within the rectangular crosshatched pattern, therebyforming a first cover plate structure from the third silicon wafersection and a second cover plate structure from the fourth silicon wafersection; (i) metallizing a rectangular pattern layer on the glass firstsurface of the first and second cover plate structures so that themetallic layer is electrically connected to the opposite silicon surfaceby the silicon mesa, and sized and positioned to coincide with the firstand second surfaces of the inertial mass; (j) bonding the glass surfaceof the first cover plate structure to the first surface of the siliconsupport structure so that the metallized rectangular pattern layer iscoincident with and spaced from the first surface of the inertial masswhereby a first variable capacitor is formed; (k) bonding the glasssurface of the second cover plate structure to the second surface of thesilicon support structure so that the metallized rectangular patternlayer is coincident with and spaced from the second surface of theinertial mass whereby a second variable capacitor is formed; and (l)providing means for electrical connecting the silicon wafer section ofthe first cover plate structure, the silicon support structure, and thesilicon wafer section of the second cover plate structure to electroniccircuitry for measuring the value of the first variable capacitor andthe value of the second variable capacitor.
 20. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, wherein the step of providing a means to detect movement of theinertial mass comprises the steps of: (a) forming a small hole in afirst glass layer and in a second glass layer at positions that coincidewith the locations of the inertial mass, each glass layer having a firstsurface and a second surface; (b) metallizing the surfaces of the smallholes; (c) metallizing a first rectangular layer on the first surface ofthe first glass layer and on the first surface of the second glass layerso that each metallic rectangular layer is electrically connected to theopposite second surface of the first and second glass layers by themetallized hole, and the metallic layers being sized and positioned tocoincide with the first and second surfaces of the inertial mass; (d)metallizing an electrical bonding pad on the second surface of the firstand second glass layers so that each bonding pad is electricallyconnected to a corresponding rectangular metallic layer on the firstsurface of the first and second glass layers by the metallized holes;(e) bonding the first surface of the first glass layer to the firstsurface of the silicon support structure so that the metallizedrectangular layer is coincident with and spaced from the first surfaceof the inertial mass whereby a first variable capacitor is formed; (f)bonding the first surface of the second glass layer to the secondsurface of the silicon support structure so that the metallizedrectangular layer is coincident with and spaced from the second surfaceof the inertial mass whereby a second variable capacitor is formed; and(g) providing means for electrical connecting the electrical bonding padon the first glass layer, the silicon support structure, and theelectrical bonding pad on the second glass layer to electronic circuitryfor measuring the value of the first variable capacitor and the value ofthe second variable capacitor.
 21. The method of manufacturing amonolithic silicon acceleration sensor according to claim 1, wherein thestep of providing a means to detect movement of the inertial masscomprises the steps of: (a) attaching piezoresistive elements to thebeam members; and (b) providing electrical connections from thepiezoresistive elements to resistance measuring circuitry to determinethe amount of twisting or bending of the beam members due to movement ofthe inertial mass in response to acceleration.
 22. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, further comprising the step of electrically interconnecting thefirst and second wafer sections of silicon by depositing a layer ofconductive polysilicon over the surface of the etched structure,following the step of stripping the etch-stop.
 23. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, further comprising the step of forming a single monolithicsilicon acceleration sensor cell for sensing planar acceleration. 24.The method of manufacturing a monolithic silicon acceleration sensoraccording to claim 1, further comprising the steps of: (a) forming amonolithic sensor comprising a first and a second silicon accelerationsensor cell for sensing two axes acceleration; and (b) orienting thesecond sensor cell at a 90 degree or a 180 degree angle to the firstsensor cell when viewing the first surface of the inertial masses, usingthe beam members as an angular reference.
 25. The method ofmanufacturing a monolithic silicon acceleration sensor according toclaim 1, further comprising the steps of: (a) forming a monolithicsensor comprising a first, second, and a third silicon accelerationsensor cell for sensing three axes acceleration; (b) orienting thesecond sensor cell at a 90 degree angle to the first sensor cell whenviewing the first surface of the inertial masses, using the beam membersas an angular reference; and (c) orienting the third sensor cell at a180 degree angle to the first sensor cell when viewing the first surfaceof the inertial masses, using the beam members as an angular reference.26. The method of manufacturing a monolithic silicon acceleration sensoraccording to claim 1, further comprising the steps of: (a) forming amonolithic comprising a first, second, third and a fourth siliconacceleration sensor cell for sensing three axes acceleration; (b)orienting the second sensor cell at a 90 degree angle to the firstsensor cell when viewing the first surface of the inertial masses, usingthe beam members as an angular reference; (c) orienting the third sensorcell at a 180 degree angle to the first sensor cell when viewing thefirst surface of the inertial masses, using the beam members as anangular reference; and (d) orienting the fourth sensor cell at a 270degree angle to the first sensor cell when viewing the first surface ofthe inertial masses, using the beam members as an angular reference.